The present invention relates to an isolating type self-oscillating flyback converter used in a self-oscillating flyback power module.
A conventional isolating type self-oscillating flyback converter includes a coupled transformer, a FET (Field-Effect Transistor), an oscillating transistor and an electro-optical coupled isolating unit. As shown in FIG. 1, after applying voltage to the input terminal, the current directly flows through resistors R1 and R2 to make the FET TR1 tend to be conductive. When there is a current flowing through the primary winding P1 of the coupled transformer, the polarity of the secondary winding P2 produced by the coupling effect makes the conductivity of the FET TR1 increase further. The positive voltage of the secondary winding P2 is initially transmitted to the FET TR1 through a speedup capacitor C1 and a resistor R2, therefore, the front edge of the waveform of the driving current is very steep, which facilitates quick conduction of the FET. After the FET TR1 becoming conductive, the current that flows through the primary winding P1 and transistor TR2 results in a voltage drop on a resistor R4, and the polarity of said resistor, which is positive at the upper end and negative at the lower end, makes the transistor TR2 become conductive through resistor R3 and capacitor C2. After the transistor TR2 becoming conductive, it makes the gate of the FET TR1 shunt, and the FET TR1 will close off. After the FET TR1 being closed off, the current flowing through its drain is zero, so that the voltage drop on the resistor R4 is zero, the transistor TR 2 will be closed off, and the FET TR1 will become conductive again. As above described process recycles, self-oscillation will be generated in the circuit. After start-up of self-oscillation, the circuit will generate flyback. The output terminal controls the current on the base of the transistor TR2 according to the feedback of the phototriode of the electro-optical coupled isolating unit, so as to regulate FET TR1 and transistor TR2. This is the current flyback process of the circuit.
Since transistor TR1 is started up directly by the voltage on resistors R1 and R2 in said circuit, the start current is quite high. In addition, when the input voltage is high and at the moment that the voltage is just applied, there will appear a very high instantaneous current spike on the resistors R1 and R2, which makes the transistor momentarily become conductive, and the current spike of this moment may strike through the transistor; meanwhile, the high voltage applied on the transistor may cause damage to the transistor, and therefore cause the bad start problem of the product.